The broad objective of the proposed research is to examine the functional adaptation of the heart and circulation to the demands of exercise and other forms of stress. Anaesthetized dogs and trained conscious dogs will be used. The areas of particular interest are a) the respective contributions of neural, humoral and intrinsic myocardial factors to the cardiac response to an increase in volume or in pressure load, b) the role of cardiopulmonary receptors in reflex control of cardiac function, peripheral circulation and circulating blood volume, and c) the degree to which the heart or the peripheral circulation each can accommodate for a disturbance in the integrated response of the other system. The ongoing research is directed particularly toward the role of the cardiopulmonary mechanoreceptors. 1) Are receptors with particular functions localized in anatomically definable regions of the heart and lungs? 2) What is the degree and extent of control exerted by these receptors over resistance and capacitance vessels? 3) What is the function of these receptors during cardiac stress or malfunction? 4) What is the role of these receptors in regard to the regulation of blood volume particularly as they might affect a) the renin - angiotensin -aldosterone system, and b) the renal handling of sodium?